ABSTRACT
In bryophytes, sexual reproduction necessitates the release of motile sperm cells from a gametophyte into the environment. Since 1856, this process, particularly in liverworts, has been known to depend on water. However, the molecular mechanism underlying this phenomenon has remained elusive. Here we identify the plasma membrane protein MpMLO1 in Marchantia polymorpha, a model liverwort, as critical for sperm discharge from antheridia. The MpMLO1-expressing tip cells among the sperm-wrapping jacket cells undergo programmed cell death upon antheridium maturation to facilitate sperm discharge after the application of water and even hypertonic solutions. The absence of MpMLO1 leads to reduced cytoplasmic Ca2+ levels in tip cells, preventing cell death and consequently sperm discharge. Our findings reveal that MpMLO1-mediated programmed cell death in antheridial tip cells, regulated by cytosolic Ca2+ dynamics, is essential for sperm release, elucidating a key mechanism in bryophyte sexual reproduction and providing insights into terrestrial plant evolution.
